Brand protection and product security can include the use of eye-catching, difficult-to-reproduce overt elements, or deterrents. The term “overt” refers to a visible or observable feature. One type of commonly used overt security element is a guilloche. Guilloche patterns are spirograph-like curves that frame a curve within an inner and outer envelope curve. These patterns are often formed of two or more curved bands that interlace to repeat a circular design, and are most commonly used on banknotes, securities, passports, and other documents as a protection against counterfeit and forgery.
Guilloche patterns can be plotted in polar and Cartesian coordinates, and these can be generated by a series of nested additions and multiplications of sinusoids of various periods. Guilloche patterns have traditionally provided an overt deterrent to copying and counterfeiting because of the difficulty of reproducing the complex patterns. In this context it is worth recognizing that overt deterrents generally rely for their effectiveness on visual detection. For an overt security element to inhibit and allow detection of forgery, a person or machine is used to notice the difference in a guilloche pattern or other complex pattern of lines (e.g. the individual lines in the portrait of George Washington on U.S. currency) in the document. In the past, forgers and counterfeiters have had to try to exactly recreate an original document or engraving by hand or other methods. Accurately reproducing a complex guilloche pattern using these methods is very difficult, and alternatives such as copying are frequently unsatisfactory due to the fine lines in the patterns.
More recently, however, the production and reproduction of guilloche patterns has been greatly simplified by the use of computer and graphics technology. Using computerized printing systems, highly complex guilloche patterns can be produced at very high resolution. Additionally, using high resolution color scanning and printing systems that are commercially available, counterfeiters and forgers can reproduce security documents in a manner that can fool all but the most trained observers. Since overt security features generally rely upon observation for detection of counterfeits, a high quality copy can be so close to the original that only an expert paying very close attention can detect the forgery.